The invention concerns an improved method of obtaining a uranyl nitrate solution by dissolving impure uranium tetrafluoride, by means of a nitric acid solution in the presence of an aluminum compound. The method comprises introducing the two reagents in two treatment stages.
It has already been known for a long time to dissolve UF.sub.4 by means of nitric acid and an agent for making fluoride ions complex, the agent comprising an aluminum compound. Thus, French Pat. No. 2,429,746 describes such a process in which uranium tetrafluoride is digested in an acid solution in the presence of such an agent for making fluorides complex, in accordance with the unbalanced equation: EQU UF.sub.4 +2 HNO.sub.3 .fwdarw.UO.sub.2.sup.2+ +2(NO.sub.3.sup.-).
The fluoride ions are made complex by the aluminum compound initially introduced, this compound being capable of forming a fluorine complex AlF.sub.6.sup.3- which is soluble in the treatment medium.
When impure UF.sub.4 is dissolved in a concentrated nitric acid solution in the presence of an aluminum compound, the medium resulting from the treatment is not the uraniferous solution expected but a suspension of fine particles, where the liquid phase contains NO.sub.3.sup.-, UC.sub.2.sup.2+, AlF.sub.6.sup.3- ions and impurities, and where most of the solid phase consists of AlF.sub.3 and nH.sub.2 O in the form of a gel. The gel has the peculiarity of retaining what may be quite a large or small quantity of uranium.
Since a suspension is obtained instead of the expected solution, it becomes necessary to separate the phases before recovering the uranium. However, such separation has proved to be extremely difficult if not impossible, e.g., in the case of separation by filtration, since the pores of filtering substrate quickly become blocked by the fine particles.